A non-topological proof for the impossibility of k-set agreement

  • Authors:
  • Hagit Attiya;Armando Castañeda

  • Affiliations:
  • Department of Computer Science, Technion, Haifa, Israel;IRISA-INRIA, Campus de Beaulieu, Rennes Cedex, France

  • Venue:
  • SSS'11 Proceedings of the 13th international conference on Stabilization, safety, and security of distributed systems
  • Year:
  • 2011

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Abstract

In the k-set agreement task each process proposes a value, and it is required that each correct process has to decide a value which was proposed and at most k distinct values must be decided. Using topological arguments it has been proved that k-set agreement is unsolvable in the asynchronous wait-free read/write shared memory model, when k n, the number of processes. This paper presents a simple, non-topological impossibility proof of k-set agreement. The proof depends on two simple properties of the immediate snapshot executions, a subset of all possible executions, and on the well known graph theory result stating that every graph has an even number of vertices with odd degree (the handshaking lemma).